In the manufacture of pulp and paper wood is usually chipped into wood particles using a chipper. Many types of chippers available, however the conventional chipper cuts across the wood at an angle to the grain to define the length of the chip and the thickness is determined by splitting along the grain. Therefore, despite the fact that major investigations have been made on cutting angles of the knives etc., the thickness of the chips produced by such conventional chippers is not accurately controlled.
Wafer chippers have also been used to produce chips for pulping, such chippers or waferizers as they are sometimes called cut generally along (parallel to) and across the grain with the main cutting edge parallel to the grain to produce chips that have a uniform thickness and therefore a more uniform impregnation characteristic. However, the benefits derived from wafer chips can only be obtained if only wafer chips are used to charge the digester. However, since it is normal practice to purchase chips from a variety of different suppliers and not all suppliers have the same type of wafer chipper the uniformity in thickness obviously is not obtained and therefore neither would the benefits. Furthermore the wafer chipper is much more expensive to maintain since it generally requires the use of a plurality of discrete knives, each of which cuts a single chip.
It has been proposed to treat chips produced by a conventional chipper to render them more uniformly impregatable for example by shredding of conventional chips to reduce them to smaller particles which may be more quickly and more uniformly impregnated, however, such shredding generally increases the number of fines which cause problems during digestion that to a degree defeat the purpose of the shredding operation.
It is also proposed to crush chips using a chip crusher such as the crusher shown in Canadian patent No. 825,416 issued Oct. 29, 1969 to Kutchers et al which utilizes a pair of rolls to crush the chips and fissure them to render them more easily and more uniformly penetrable by cooking liquor in the pulping process.
In the said Kutchera et al patent a specific surface design is proposed wherein each of the rolls are provided with ribs spaced 0.375 to 0.91 inch and have uniform heights between about 0.007 and 0.13 inch, and surfaces or land areas of 0.12 inch to 0.2 inch with the rib height ratio of the two rolls never exceeding about 4 to 1.
The device of the Kutchera et al patent has been tried but it is believed it is no longer in operation, part of the problem being the limited capacity of the equipment.
U.S. Pat. No. 3,962,966 issued June 15, 1976 to Lapointe describes an improved arrangement for increasing the throughput through the crusher. In this device the chips are fed axially onto a rotating disc which flings them out radially in a substantially one chip thickness layer, that passes between a roll and a working surface of the disc to squeeze chips of greater than a certain thickness.